The present invention relates to silicone compositions for release paper which provides excellent releasability and slip properties with substrates, more particularly to a solventless silicone composition for release paper which may be applied without containing organic solvents.
Conventionally, silicone compositions have been used as release agents to prevent the adhesion and fixing between substrates such as paper or plastics and adhesive substances, or to improve the slip between paper and paper, paper and plastics, a plastic film and a plastic film, paper and human fingers, plastics and human fingers, or plastics and metals.
These release agent silicone compositions have been widely used as organic solvent solutions or as aqueous emulsions (see, for example, Japanese Patent Publication Nos. Sho 35-13709, Sho 36-1397, and Sho 46-26798). Since the organic solvent solution-type silicone release agents contain, as a main constituent, gum-like diorganopolysiloxanes of a high molecular weight or of a high polymerization degree, they need large volumes of solvents in their use, and thus inevitably cause air pollution if the solvents are vented into the atmosphere, and they require a substantial cost in the solvent recovery system. In contrast, the aqueous emulsion-type silicone release agents cause no serious air pollution; however, they might cause some environmental pollution from the use of emulsifying agents, and they are required to be heated for a longer time at a high temperature for film formation.
To solve these problems, solventless-type release agent silicone compositions have been proposed which contain virtually no water or organic solvents. Examples of these silicone compositions may be found, for example, in U.S. Pat. Nos. 3,922,443, 4,057,596, and 4,071,644, and Japanese Patent Publication No. Sho. 52-39791. However, the cured film obtained from these compositions has a low polymerization degree, because these compositions use a vinylsiloxane or hydrogensiloxane having a viscosity between 50 and 5,000cSt at 25.degree. C., as a main constituent, and as a result it is inferior in mechanical strength in comparison to that obtained from the conventional organic solvent solution-type silicone compositions stated above. In addition to this disadvantage, it is unsatisfactory in slip properties, and thus the conventional solventless silicone compositions have limited applications.
For example, if the conventional solventless release agent silicone compositions are used for release agents for a kraft tape with a pressure-sensitive adhesive, the application of the kraft tape to corrugated boards is not smoothly done, since the cured film formed on the back side of the kraft tape is poor in slip properties, and the slip between hands and the release surface is not good. Sometimes workers hurt their hands. If this happens, workers cannot apply sufficient force to the kraft tape, and thus the pressure sensitive adhesive does not satisfactorily adhere to the corrugated board, and thus the peeling of the tape from the corrugated board sometimes happens. It has also been pointed out that in case the conventional solventless release agent silicone compositions are applied to paper or a plastic film to make a slip sheet for a cassette tape, a magnetic tape is sometimes damaged, or troubles occur in tape rotation, because the slip between the tape and the slip sheet is not good.
It has further been pointed out that if release paper is manufactured by applying and curing the conventional solventless release agent silicone compositions to paper or a plastic film, the cured silicone film thus obtained contacts a metal or plastic roll before the release paper is wound up. The poor slip properties of the cured film might cause damages to the silicone surface, and this becomes a cause of scattering in release performance of the release paper thus manufactured. The poor slip properties also might make the wind-up of the coated paper or plastic film unsmooth, and thereby cause processing troubles.
To solve these problems in the conventional solventless release agent silicone compositions, various slip-imparting materials, such as dimethylsiloxane oils, polyether modified oils, surfactants, waxes, and metal soaps, have been added. However, satisfactory results have not yet been obtained. Since virtually none of these slip-imparting materials are very compatible with the silicone compositions, the coating fluid thus obtained becomes unstable, and curing often fails to occur. The most serious problem is that almost all of these slip-imparting materials tend to migrate onto the surface of the silicone cured film, and this causes a decrease in the adhesive force between the cured film and the adhesive.
Other methods for improving slip properties have been proposed, as for example, in Japanese Patent Laid-open Publication Nos. Sho. 61-159480 and Sho. 63-101453. However, these methods are still unsatisfactory.